1 Place the onions, garlic, vinegar, oil, jam, salt, and red pepper in a self-sealing plastic food storage bag. Push out all the air and seal the bag, then knead the marinade through the bag until it is combined. Using a sharp knife and cutting a scant 1/8 inch deep, score the steak on both sides in a diamond pattern. Add the steak to the marinade, coat the steak well.and reseal the bag. Place the bag on a plate and marinate the steak in the refrigerat or for at least 30 minutes or up to 3 hours.
2 Preheat the broiler, setting the rack 5 inches from the heat. Place an 8-inch square of aluminum foil on one end of the broiler pan. Using a slotted spoon, remove the onions from the marinade and arrange them on the foil. Place the steak directly on the pan next to the onions. Broil the onions for 5 minutes on each side, and the steak for 5 to 6 minutes on each side for medium-rare, 7 to 8 minutes for medium.
3 Transfer the steak to a cutting board and let stand for 10 minutes; then thinly slice it against the grain, holding the knife at a slight angle. Place the slices on individual plates and spoon any juices over them.